We have been discussing two groups of potential killers, jealous exes (crime of passion) and incels who are hating all women. But there is a third group, not too popular, repressed guys with whom popular girls could have very brief flings. It happens; either the girls want to evoke jealousy in their exes, or just was in the cards. And then the guy is all ready for a relationship, but the girl soon gets back together with her BF.
IMHO, this could cause so much hatred, especially if a guy feels that he was used. And the truth is, this is the group that could totally fly under the radar. Either no one noticed because it was too short, or the one who could remember is dead, too.
I think it could happen with any girl, and maybe not disgruntled exes but potentially, these very brief relationships should be in the focus. Of course exes can commit crimes of passion, but likewise, they have something to hope for. Of incels, I think the biggest loners could stand out, not everyone. But “in love, then hopeless” guys can be very dangerous, JMO.